Field of the Invention
The present technology relates to high density memory devices based on phase change based memory materials.
Description of Related Art
Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.
In phase change memory, data is stored by causing transitions in an active region of the phase change material between amorphous and crystalline states. For a one bit cell, the difference between the highest resistance R1 of the low resistance crystalline set state and the lowest resistance R2 of the high resistance amorphous reset state defines a read margin used to distinguish cells in the crystalline set state from those in the amorphous reset state. The data stored in a memory cell can be determined by determining whether the memory cell has a resistance corresponding to the low resistance state or to the high resistance state, for example by measuring whether the resistance of the memory cell is above or below a threshold resistance value within the read margin.
Phase change materials used for memory operations can be characterized by a number of different performance specifications including set and reset speeds, data retention, endurance, reset current and the probability that data stored using the material will survive solder bonding. The speed is a function of the pulse lengths required to cause the set and reset operations, as well as other factors. The data retention is a function, typically, of the tendency of the phase change material in the amorphous phase to crystallize over time and temperature so as to lose data. The endurance is a function of the phase change material tendency to become hard to set or hard to reset as the material is exposed to many set/reset cycles. The reset current has a value which is desirably low, but must be sufficient to cause heating in the active area sufficient to cause the phase transition, and can be higher as the resistance level of the set state is lower. The solder bonding specification refers to a difficulty that arises with phase change materials on integrated circuits that are mounted by solder bonding. The solder bonding process exposes the integrated circuit to elevated temperatures, which can exceed the crystallization transition temperature for the phase change materials. Thus, prior art phase change designs could not be programmed before being mounted by solder bonding.
It is desirable therefore to provide a phase change memory material with a higher crystallization transition temperature to prevent undesired transformation from the amorphous reset state to the crystallized set state at elevated operation temperatures. It is further desirable that the phase change memory material maintain a large difference between the range of reset state resistance values and the range of set state resistance values. It is also desirable that the phase change memory material retains fast set and reset speeds. It is also desirable that a phase change memory material have relatively higher resistance in the crystallized set state phase, to reduce the reset current. Furthermore, it is desirable that a phase change memory material have a crystallization transition temperature high enough to withstand solder bonding without loss of data.